The present invention relates in general to clutches for driving units and, in particular, to a new and useful planer drive mechanism with a clutch formed between a secondary and a primary rim.
As is well known, a friction clutch is an overload clutch which slips as soon as the transmitted torque exceeds a maximum of the friction torque.
According to German Pat. No. 2,702,178, a friction clutch is provided in drives for planer sprocket wheels instead of the usual shearing safety bolts, so that operating problems caused by broken safety bolts are avoided. The same goes for chain shocks transferrred to the adjacent part of the transmission upon safety bolt ruptures. The friction torque is predetermined by the depth or degree of engagement of the tapered heads of the coupling bolts in the conformable circular groove of the sprocket wheel rim. The centering ring, which is freely mounted within the primary rim, provides for a uniform contact pressure of the coupling bolts and for a pressure equilibration by elastic deformation. In this prior art, however, the friction clutch, the coupling bolts and their tapered heads form an integral unit, i.e. the heads are formed by beveling the bolt shank end. The size, and thus the area of the frictional surfaces, are determined by the diameter of the shank, and the bolt heads are necessarily made of the same material as the coupling bolts themselves, wherefore, they have a relatively low coefficient of friction. The limited friction areas result in a need for relatively high specific contact pressures, causing an increased wear. The wear is aggravated by the fact that the tapered bolt heads simultaneously produce a shaving effect within the circular groove, and the coefficient of friction is predetermined by the material employed for making the coupling bolts.